Compatibility of Porous Silicon with Lithium Polysulfides for Full Cell Lithium Ion Battery Applications with Sulfur Cathodes

摘要

Crystalline silicon is an ideal anode candidate for lithium ion batteries as it offers a high Li~+ ion storage capacity, direct current pathway and is a stable and earth abundant element. In order to match the high theoretical energy density of silicon in lithium ion batteries a suitable cathode material is required. Sulfur based cathodes have a high theoretical storage capacity for application in lithium ion batteries, based on the conversion reaction of the cyclic S_8 molecule with lithium to form smaller lithium polysulfides up to Li_2S, and are also cheap and earth abundant. One of the major drawbacks of lithium sulfur cathode materials is the tendency of lithium polysulfides to be dissolved by the electrolyte, which causes the polysulfides to migrate away from the cathode, leading to an overall capacity loss. For application of porous silicon in a full cell lithium ion battery with sulfur it is important to understand the impact the presence of lithium polysulfides can have on the functionality of porous silicon as an anode. In this study porous silicon anodes with thicknesses of 6 μm and 12 μm were cycled in a half cell in the presence of varying concentrations of Li_2S_6 (0 M, 0.017 M and 0.083 M). For the electrolyte with 0.017 M Li_2S_6 the cycling behavior was comparable, or better, than the electrolyte with no polysulfides. The SEI layer formed on the surface of the porous silicon anodes was investigated ex situ with X-ray photoelectron spectroscopy, Raman spectroscopy and scanning electron microscopy after one, five and ten cycles. It was seen that in the first cycle lithium polysulfides are contained in the SEI and undergo an oxidation reaction to form sulfates over the following cycles. This leads to a more inorganic SEI layer in the presence of polysulfides, especially in the case of the 12 μm porous silicon, and long term cycling stability. The 12 μm porous silicon anode cycled in the presence of 0.083 M Li_2S_6 showed stable cycling behavior over one hundred cycles, with limited capacity loss and coulombic efficiencies of 96.2% after one hundred cycles.